1. Field of the Invention
The present invention relates to a tunable filter which is able to select a desired wavelength in a light signal including a plurality of wavelengths and which is able to change a wavelength to be selected.
2. Discussion of the Related Art
In an optical communication network using a wavelength division multiplex communication technology (WDM), expansion of a transmission capacity is realized by multiplexing a plurality of wavelengths of lights at wavelength intervals determined by the International Telecommunication Union (ITU). A conventional WDM communication realizes a wavelength multiplexing at an interval of 0.8 nm, for example, in a wavelength band of 1550 nm, and now introduction of a system having a narrow wavelength interval, for example, 0.4 nm or 0.2 nm in further narrow wavelength band is considered. A wavelength-tunable filter is used for selecting a signal of arbitrary wavelength from a multiplexed WDM signal. In addition, the tunable filter is also used in an optical measurement apparatus and a wavelength-tunable laser.
As the tunable filter, a dielectric multilayer filter made by in turn stacking a plurality of materials with different refractive indexes, an optical fiber grating that is a diffraction grating made by in turn arranging cores with low refractive index and with high refractive index in a core of an optical fiber, and a Fabry-Perot etalon using a multiple interference between two reflectors are used.
Widely used in a WDM transmission system is the dielectric multilayer filter where a shape of the filter can be easily controlled. In a case of changing a selected wavelength by using the dielectric multilayer filter, Japanese Unexamined Patent Publication No. 05-281480 shows a method for tuning a wavelength by changing an incidence angle to the dielectric multilayer filter based on changing an incidence angle to a multilayer membrane filter or on changing a thickness of the dielectric multilayer filter to a predetermined axis direction. In addition, Japanese Unexamined Patent Publication No. 2004-177605 shows a configuration of the tunable filter which uses the incidence light as emitted lights arranged in a belt-shaped form for every wavelength and which selects and reflects the light of one wavelength among this emitted lights by using a mirror.
However, in Japanese Unexamined Patent Publication No. 05-281480, the narrower the wavelength is, the larger the total number of membranes in the multilayer membrane is required to be, and thus it is difficult to manufacture a filter with a desired wavelength distribution by accurately producing membrane thicknesses and refractive indexes based on design values.
In Japanese Unexamined Patent Publication No. 2004-177605, a wavelength is selected by moving a mirror and a shielding body to a distribution direction of light, however, it is difficult to move the mirror and the shielding body to a predetermined position accurately.